Decarburization is a change in the structure and content of steel in which some of the carbon in the surface layer or layers of the steel is lost. In total decarburization, the upper layer of the steel is composed primarily of ferrite materials, while in partial decarburization, a mixture of materials may be present. Microscopy can be used to identify carbon loss, and other testing techniques are also available.
In some cases, decarburization may be deliberately accomplished. In other instances, it's a byproduct of corrosion or poor handling techniques. Classically, decarburization occurs when steel is heated in an environment where oxygen is present, leading to oxidation and loss of carbon. As a result of decarburization, the metal loses some of its strength and ductility, and it may develop cracks which make it vulnerable to breaking. The surface of the steel may also become scaly.
When decarburization is viewed as a defect, materials testing is used on steel to confirm that the level of carbon loss is acceptable. If it is not, the steel will not be used. Loss of carbon can make structural steel less stable, erode the performance of steel tools, and cause a variety of other problems with equipment made from steel. In some settings where it is deliberately desired, testing can also be used to determine which level of decarburization has been achieved.
The rate of decarburization can be controlled. If it is not desired, protective coatings can be used on the steel to prevent corrosion, or it can be heated in an inert atmosphere which lacks oxygen. A number of systems to control carbon loss are available through companies which make equipment used in processing and working steel, and it is also possible to develop a custom system for a specific need. Conversely, the flow of oxygen into a furnace can be controlled to achieve a desired level of carbon loss.
When steel components fail, as when a bridge collapses, materials testing can be used to learn more about the steel and to identify any problems which could have contributed to the fail. Decarburization can be involved in the failure of steel components, and it leaves a clear signature behind which makes it hard to miss. In the process of identifying the cause of a problem, inspectors may also be able to determine who is responsible. For example, a construction company might have used substandard steel to cut costs.